Since first reported in 1967, optically transparent thin layer electrodes (OTTLEs) have been used for thin layer studies. Spectroelectrochemistry, for example, is a combination of electrochemical and spectroscopic techniques in which optical measurements are referred to the potential of a working electrode. Thin-layer spectroelectrochemistry is possibly the simplest type of spectroelectrochemistry and has advantages such as rapid and exhaustive electrolysis and small volume features. A typical application of an OTTLE is the spectroscopic study of “redox” processes; i.e., reactions in which the acceptance of an electron (reduction) by a material is matched with the donation of an electron (oxidation). Various spectroscopic techniques such as luminescence spectroscopy, Fourier transform infrared (FTIR) difference spectroscopy and ultraviolet/visible/near infrared (UV/vis/NIR) have been coupled with electrochemical techniques via OTTLEs. A variety of OTTLE designs for many purposes have been developed, and all generally operate on transmittance principles.